Friday, May 30, 2008

Green algae are photosynthetic microorganisms capable of using protons as a reductant and producing molecular hydrogen. As technology advances, these organisms might provide an efficient, cost-effective method to mass produce hydrogen gas to be used as a renewable source of energy. Currently, hydrogen fuel is extracted from natural gas and other non-renewable energy sources, which release particulate matter and greenhouse gases into the atmosphere during their extraction and processing. Algal biosynthesis of hydrogen is particularly promising because it uses two of Earth’s most abundant resources, light and water, to form an "eco-friendly" biofuel.

Hydrogen gas production by green algae is a consequence of anaerobiosis, which forces the cell to rely on molecular hydrogen as a reductant. Green algae are exposed to anaerobic conditions in lake and sea sediments, which can become anoxic with insufficient water turbulence or excessive algal blooms. Historically, hydrogen production by green algae was induced by anaerobic incubation in the dark, which stimulates the expression of a hydrogenase enzyme.

The major roadblock for commercial application of hydrogen biosynthesis by algae is the fact that algal hydrogenases are inhibited by molecular oxygen. This sensitivity to oxygen has motivated extensive work to genetically engineer mutants of Chlamydomonas reinhardtiiwith more oxygen tolerant hydrogenases. The successful engineering of a more oxygen tolerant hydrogenase would put us one step closer to commercial biosynthesis of molecular hydrogen for fuel. Another approach to the problem of hydrogenase oxygen sensitivity is temporal separation of the water splitting and hydrogen evolving reactions.

Researchers have also shown that sulfur deprivation improves hydrogen yield by inhibiting molecular oxygen evolution. In the absence of necessary nutrients, metabolism and growth slow significantly too conserve the remaining substrates. In the case of green algae and other photosynthetic organisms, sulfur depleted environments stimulate the downregulation of photosytem-II and consequently, molecular oxygen evolution. Under these circumstances, electrons are not sufficiently removed by molecular oxygen. Green algae significantly increase hydrogen production under sulfur-deprived conditions because hydrogenase is charged to remove the excess electrons.

Hydrogen fuel offers an efficient, environmentally friendly alternative to gasoline and biodiesel. Hydrogen is a potent, cost-effective fuel because it has the highest energy content per unit of weight of any known fuel. Hydrogen gas production by green algae shows enormous promise, but requires a few manipulations to make the process feasible on a commercial scale. To improve this process, the number of hydrogenase expressed in the cell must be increased (without being toxic) to increase the yield of hydrogen per algal cell. The oxygen sensitivity of the hydrogenase enzyme must also be reduced so that the enzyme can produce hydrogen more efficiently in easily managed environments. It is highly likley that within our lifetimes, we will see algal hydrogen production on a commercial scale.